Ch 3: Installing Optional Devices
Hitachi PC VisionBase8450H/R Server 55
Functions of Disk Array
Degraded mode
The RAID3 or RAID5 disk array enables you to keep on using the array as usual even if one of the hard disks fails.
When a disk array operates with one failed hard disk, the operation is referred to as degraded-mode operation.
Whena read or write operation is requested, the array disk controller computes data in the failed disk based on the
data in the active disks and array parities. However, because of this data computation, processing efficiency
decreases as compared with usual situations.
Rebuilding data
In the RAID1, RAID3, or RAID5 disk array, after a failed disk is replaced, the array controller automatically restores
data and stores it in the new disk based on the data in the mirrored disk and parity data. This is called “rebuilding
data”. You can keep on your task while data is being rebuilt, but processing efficiency decreases, as in degraded
mode.
During a degraded-mode operation, an additional hard disk failure will disable the disk array to rebuild data,
resulting in loss of all data. Be sure to replace the failed disk immediately and rebuild data.
NOTE: The RAID0 disk array cannot rebuild data because it does not dualize data nor create parities.
Hot plug
When the disk array operates in degraded mode, you need to replace the failed disk as early as possible. However,
turning off the system equipment is inconvenient because it is time-consuming and you need to stop the operation.
The function of “hot plug” (also called “hot swap”) is the solution to this problem. With this function, you can
replace hard disks while the system equipment is powered.
Hot spare
If the RAID1, RAID3, or RAID5 disk array fails, you need to replace the failed disk in order to recover to the normal
status. By installing a reserve disk (with no data) in the disk array, you can have data automatically restored to the
reserve disk. This reserve disk is called “hot spare” because it is provided for replacement at any time. The array
controller’s function of automatically restoring data to the hot spare is called “standby replacement”.
